This invention relates generally to apparatus for elevating personnel and/or loads utilizing scissor-type linkage.
Aerial work platforms generally comprise a wheeled base, which is normally self-propelled, and a platform which may be raised or lowered relative to the base by means of scissor-type linkage arms. Typically, these scissor-type arms are hydraulically powered to be moved between a folded, lowered position to an extended, raised position, with the height of the work platform being varied as desired by the operator.
Providing sufficient hydraulic power to raise and lower the platform requires the provision of lift cylinders which are actuated to apply a force to the arms thereby moving them between the folded and extended positions.
A limitation in providing sufficient hydraulic power relates to the overall height of the platform when the arms are folded on the base. As low a height as possible is desirable so that the apparatus can be moved into and from areas having restricted entryways. Vertically mounted hydraulic cylinders obviously offer the most efficient mechanical advantage. This arrangement is shown in U.S. Pat. No. 4,194,723, assigned to the assignee of the present invention. Due to height restraints, such vertically oriented cylinders are not always acceptable or desirable. As an alternative, the cylinders are usually mounted at some angle relative to the arms, which reduces the overall vertical height with respect to the folded platform. However, a result of such angular mounting is a loss of mechanical advantage of leverage since the folded arms and the cylinder are connected so as to form a very small acute angular relationship. In fact, the cylinders and arms are frequently almost parallel when the lift arms are retracted. This requires greater hydraulic force to raise the platform. After the arms are partially extended and the cylinder is pivoted to a more vertical position, the leverage problem is reduced.
An attempt to overcome this problem is disclosed in U.S. Pat. No. 4,114,854. In the lift platform described, a cylinder moves a pivot arm upwardly toward the lift arms. The pivot arm co-acts with a cam connected to the scissor lift arms. The resultant force provides a slight mechanical advantage over connecting the cylinder directly to the scissor lift arms, but the vertical force component is still not as large as desired.
The foregoing illustrates typical limitations known to exist in present lift arrangements. Thus, it is recognized that it would be advantageous to provide a more efficient lifting system for scissor lift equipment, and the present invention is believed to meet that need.